Title: Coherent phase stability in Al-Zn and Al-Cu fcc alloys: The role of the instability of fcc Zn

The coherent phase stability of fcc-based Al-Zn and Al-Cu alloys is studied theoretically by first-principles total energy calculations, a mixed-space cluster expansion approach, and Monte Carlo thermodynamic simulations. We find that a large portion of the differences between Al-Zn and Al-Cu can be explained by the differences between fcc-Zn and fcc-Cu: While Zn is stable in the hcp structure, fcc-Zn shows an instability when deformed rhombohedrally along (111). In contrast, fcc-Cu is the stable form of Cu and is elastically extremely soft when deformed along (100). These elastically soft directions of the constituents permeate the phase stability of the alloys: (111) superlattices are the lowest energy coherent structures in Al-Zn, while (100) superlattices are stable coherent phases in Al-Cu. The short-range order of both Al-rich solid solutions show clustering tendencies, with the diffuse intensity due to short-range order in Al-Zn and Al-Cu showing streaks along (111) and (100), respectively. The mixing enthalpies and coherent phase boundaries are also calculated and found to be in good agreement with experimental data, where available. (c) 1999 The American Physical Society.